This invention relates to a motor drive circuit which drives a motor load using a DC source, and which when required to stop the motor load, brakes the same.
A conventional motor drive circuit of this type is used, example, for in driving a vehicle wiper motor. Such a motor drive circuit is shown in FIG. 13. In FIG. 13, reference numeral 1 designates a vehicle battery, the positive and negative terminals of which are connected to a series circuit of a pnp transistor 2 and an npn transistor 3; and diodes 4 and 5, which are connected in parallel to the transistors 2 and 3, respectively. These diodes bypass current when voltage is applied to transistors 2 and 3 in the reverse direction. Further in FIG. 13, reference numeral 6 designates a wiper motor the two terminals of which are connected to the common connecting point of transistors 2 and 3 and the negative terminal of the battery 1; and 7, a control circuit for applying control signals to the bases of transistors 2 and 3 to the on-off switching control of the transistor.
The motor drive circuit thus constructed, and in particular control circuit 7 operates to render transistor 2 conductive (ON), such that current is supplied from vehicle battery 1 to wiper motor 6 to drive wiper motor 6. To stop the wiper motor, the control circuit 7 operates to render transistor 2 non-conductive (OFF) to stop the flow of current from the vehicle battery 1 to wiper motor and to render the other transistor 3 conductive (ON) to substantially short-circuit the two terminals of the wiper motor 6, thereby braking the wiper motor 6.
The above-described conventional motor drive circuit is disadvantageous in that if vehicle battery 1 is connected with the polarity reversed, then forward voltage is applied to diodes 4 and 5 connected in parallel with transistors 2 and 3, such that a large current flows through diodes 4 and 5 breaking them. This difficulty may be eliminated by removing diodes 4 and 5 from the circuit. However, this is still disadvantageous in that transistors 2 and 3 may be broken if a negative surge voltage inherent in such vehicle system is produced. Accordingly, transistors 2 and 3 must be rated to withstand high voltage, which increases the manufacturing cost of the motor drive circuit.
The above-described difficulties have been overcome in-part by a motor drive circuit design as shown in FIG. 14. In this motor drive circuit, a reverse current blocking diode 8 is connected between the positive terminal of the vehicle battery 1 and emitter of transistor 2 (with the diode 4 from FIG. 13 eliminated). With the motor drive circuit thus constructed diode 8 eliminates the above-described difficulties accompanying the case wherein battery 1 is connected reverse in polarity.
While it is true that the motor drive circuit of FIG. 14 can prevent this difficulty the circuit is nevertheless disadvantageous in that while wiper motor 6 is being energized by vehicle battery 1; i.e., during the operation of wiper motor 6, a load current flows through the reverse current blocking diode 8 at all times. Therefore, if the load current is large, diode 8 is greatly heated, thus increasing its temperature. Accordingly, the diode must be rated for large capacity, and it is necessary to provide a heat radiation board to help suppress the increase in temperature. Thus, unavoidably the motor drive circuit of FIG. 14 is relatively expense to manufacture and bulky.
The above-described difficulties may be overcome by the provision of a motor drive circuit as shown in FIG. 15. In this circuit, a reverse current blocking diode 9 is connected between vehicle battery 1 and transistor 3, to eliminate the difficulties which are caused if vehicle battery 1 is connected reverse in polarity. In the motor drive circuit of FIG. 15, the forward current flows through diode 9 only when braking wiper motor 6, and accordingly the period over which diode 9 is heated is greatly reduced. Thus, the motor drive circuit shown in FIG. 15 is less costly to manufacture and can be miniaturized.
However, the with motor drive circuit with having the reverse current blocking diode 9 as shown in FIG. 15 is not practical in use, because when the application of current to wiper motor 6 is suspended because it is locked, the inductance component of the wiper motor induces a great counter electromotive force, or surge voltage (for example -150 to -160 volts) across the two terminals of wiper motor 6. Since the motor drive circuit shown in FIG. 15 has no surge voltage discharging path, the surge voltage is, thus, applied across the emitter and collector of the transistor 2, thereby braking transistors 2 and 3.
In each of the motor drive circuits shown in FIGS. 16, 17 and 18, the terminals of wiper motor 6 are connected to the common connecting point of transistors 2 and 3 and the positive terminal of vehicle battery 1. That is, in the case where the operating functions of transistors 2 and 3 are exchanged one for other with the terminals of wiper motor 6 connected to the common connecting point of transistors 2 and 3 and the positive terminal of vehicle battery 1, the above-described difficulties still arise in the manners previously described.